1. Field of the Invention
This invention relates to an electronic device for generating special effects on color television pictures by modifying the luminance signal of a video signal, and particularly for inserting titles on color television pictures. In the following description, by video signal is meant a coded color television signal comprising a known double logic and analog coding. The analog part transports the luminance and chrominance signals and the logic part transports the synchronization signals.
2. Description of the Prior Art
One of the related systems in the prior art is the SECAM color television process designed for the purpose of superposing the title pictures on color pictures; in this system the luminance signal and the chrominance signal making up the video signal are separate signals. The video signal is subjected to two different frequency filters in first and second parallel channels. The first low frequency filter extracts the low frequency luminance part transmitted by the first channel. A second filter complementary to the first extracts on the second channel all of the high frequency signals of the video signal surrounding the frequency spectrum of the chrominance signal. The luminance signal of the title pictures is then added to the luminance signal of the video signal. In this system, in order to avoid exceeding the amplitude of the separate luminance signal, a maximum value is required by video signal transmission and the luminance signal emitted from the preceding signal addition step must then be clipped. The chrominance subcarrier in parallel is then attenuated in the second channel. The luminance and chrominance signals respectively emitted from the outputs of the two parallel channels are then mixed in order to form the video signal to transmit the SECAM special effects titled color pictures.
Apart from the error introduced in clipping, this method entails possible errors in phasing of the luminance and chrominance signals. The composite output video signal is not substantially identical to the composite input video signal because precise complementarity of the low-pass and high-pass filters is particularly difficult to obtain.
Also, it is recognized that the amplitude of the video signal luminance signal which issues from the mixer of a color television studio sometimes exceeds the maximum permissible value Y.sub.M, which is roughly equal to 0.7 V, corresponding to a maximum white picture.
In order to overcome these errors, the SECAM system devices limiting, at a predetermined nominal value the Y.sub.M of the white light corresponding to the amplitude of the luminance signal part having a frequency spectrum less than that of the chrominance signal, and devices to separate the input channel of the video signal in a main channel and filter channels which restore only the above-mentioned part of the luminance signal. These correcting devices are described in German Pat. Nos. 1,562,026 filed Feb. 15, 1968 and 2,048,045 filed Sept. 30, 1970.
Generally speaking, these devices comprise an input stage, a delay network inserted in the main channel which delays the input video signal for a duration which depends on the filtering and in the filtering channel according to the following order a low-pass filter connected to the output of the input stage as the delay network, the frequency spectrum of the output signal of the low-pass filter being the part of the luminance signal spectrum lower than the chrominance signal spectrum, an amplitude limiter or clipper at the nominal value of the white light, a phase inverter of 180.degree. of the luminance signal and summation device operating as an adder whose inputs are connected to the outputs of the phase inverter and delay network. In this way, the adder tansmits the video signal limited to the nominal value of white light.
However, electronic devices of a known type inserted in the filtering channel and described in the above-mentioned patents, induce interference signals particularly in the chrominance signal frequency band thereby generating considerable disturbance during transmission of the output video signal. Also, these devices do not allow for easy use of the switching signal control when they are to be used for generating special effects on color television pictures.